A. Field of the Invention
The present invention relates generally to the field of ophthalmology, and more particularly to the field of automatic optometers and lensmeters for measuring refractive characteristics of a refractive optical system.
B. Description of the Prior Art
Automatic refractors for objectively measuring refractive properties of an eye, and automatic lensmeters for measuring refractive properties of lenses, are known to operate by introducing a beam light to the particular refractive optical system to be measured and detecting its influence on the beam due to refraction. In the case of objective refractors, also referred to as objective optometers, it is known to provide a light source and a plurality of optical elements defining an illuminating optical path to the eye, and a matching plurality of optical elements defining a detecting optical path from the eye to a detector, whereby all or part of an image of the light source formed on the retina is detected through retro-reflection on the detector. The vergence of the illuminating beam is varied by moving certain of the optical elements of the optometer through a range of positions to cause focusing of the light source by the eye at different locations relative to the retina, so that an optometer position where a "best focus" occurs at the retina can be found. As will be understood by those familiar with the art, the optical elements define a Badal system. The vergence of the illuminating beam is expressed in terms of diopters. In this way, the amount of spherical refractive error is determined. A similar "best focus" procedure is followed for determining the amount of cylinder refractive error and its angular orientation or axis by rotating pairs of oppositely powered cylinder lenses about the illuminating and detecting optical paths.
Heretofore, automatic objective optometers have generally relied on principles of repetitive measurement, whereby the optometer position at which a maximum detector signal is received is recorded over and over again for repeated measurement cycles of the instrument with respect to the same eye, and a statistical evaluation of the results is performed to reach a single overall value for the best focus optometer position. This approach involves substantial complexity, and it ignores signal information available throughout the full range of optometer positions and the expected behavior of same. Moreover, modulative noise due to individual sensitivity of detector elements (pixels) remains a factor which affects measured values.